Currently in the "AuthenticationMiddleware" class when processing the External Authentication plugins, it has a try catch block around the operation. The catch block is empty... (lines 50-60 in version 4.2)
I spent a few hours tracking down an issue with my sso interface as the "AuthenticationMiddleware" class was eating an error produced by my plugin. For this reason I think it would be a great feature to add a flag on the External Authentication configuration screen to enable additional logging or reporting of errors when executing the middleware.
I am only asking for the flag, as feel the code from NopCommerce is great quality, and it would not have been left blank accidentally. If I am wrong and logging the error all the time will not cause issues, That would be great as well.